Delphinium yunnanense is an endemic species in the Yunnan Province of China, while Delphinium elatum ‘Guardian Blue’ is widespread as cut flowers in the world. They all bloom attractive blue flowers, which are a crucial ornamental trait for floriculture. In this experiment, the cross-compatibility between D. yunnanense and D. elatum was characterised and evaluated. The results showed that a strong cross-incompatibility was observed in D. yunnanense × D. elatum, resulting in hybrid sterility. The F1 hybrid generation produced neither fruits nor seeds. The cross-incompatibility of the hybrids resulted from the disturbed structure of the pollen tubes penetrating pistil style and callose deposits on stigma and in style. Moreover, the germination rate of D. elatum × D. yunnanense F1 seed was lower than their own allogamic crosses, although there was no difference in fruit set rate. The results confirmed the cross-incompatibility between D. yunnanense and D. elatum, suggesting that D. elatum is more suitable as a maternal parent for hybridisation breeding. Our study characterised and evaluated the interspecific cross-compatibility through the pollen–stigma interaction and the reproductive system in Delphinium, which could provide the basis for the development and utilisation of potential ornamental traits in Delphinium.
